In theory, the purchase of carbon offsets is supposed to cancel out the emissions generated by activities like flying or heating office buildings by directing money to programs that reduce emissions elsewhere, like tree-planting in Africa or a hydropower project in Brazil. An airline passenger might volunteer to pay $5 to $40 to offset his flight, with the price linked to distance.

Offsets have played a growing role in the greening of travel because carbon dioxide emissions from airplanes are growing so quickly and there is currently no technological fix that would drastically lower them.

In the United States, dozens of hotels and airlines have embraced such programs in the last year or two. United Airlines became the latest American airline to offer one this summer. Globally, offset programs have grown into a multimillion-dollar industry.

But it has proved difficult to monitor or quantify the emissions-reducing potential of the thousands of green projects financed by customers' payments, and there are no industrywide standards.

Responsible Travel is not the only organization that has changed its mind about the usefulness of offsets: Yahoo and the U.S. House of Representatives both ended trial offset-purchase programs this year, concluding that the money was better spent on improving their buildings' energy efficiency.

Some of the world's leading experts on the emissions issue have reviewed and rejected purchasing offsets for air travel.

"We're always looking at it, but so far I've decided not to do it," said Paul Dickenson, chief executive of the Carbon Disclosure Project, a vast nonprofit consortium of companies that have pledged to report and reduce their emissions. For one thing, he said, offsetting the emissions of a flight from London to New York would probably require an extra fee of $200 to $300, far above what any airline is now charging.

And some experts say that emissions from airline travel are simply so large that it may be impossible to offset them.

"Buying offsets is a nice idea, just like giving money to a soup kitchen is a nice idea, but that doesn't end world hunger," said Anja Kollmuss, a staff scientist for the Stockholm Environment Institute who is based at a branch at Tufts University.

"Buying offsets won't solve the problem, because flying around the way we do is simply unsustainable," said Kollmuss, who has researched airline offsets.

No fucking shit, Captain Obvious! And fuck all of the Gaia Worshipping Enviroweenie Hippie Goatse Gloryholing Earfuckers up their unwashed asses with the jetblast from Algore's luxury plane for tryning to push this rubber fist of FAIL bullshit upon us. This shit was never anything more than a fucking attempt to assuage the limousine liberal guilt for being able to hop on a plane or drive a car on halfway paed roads and to enrich hucksters such as Algore and the stock brokers who sold these abominations.

And you know what, Buttboy of Birkenstock? I fucking hope you drained your bank accounts to pay for these stupid "offsets", because a fool who believes in that shit should not be allowed any more than a daily allotment of lunch money to spend. I fucking hope you realize what a sucker you were, and then I shall laugh the belliest of Errol Flynn belly laughs in your face. And then I will burn some cans of hairspray on your lawn to taunt you.

Now fuck off, Green Goofs, and leave reality to those of us who have a fucking clue.